Pay per click advertising is an arrangement where a web publisher displays a clickable advertisement in exchange for a charge per click paid by the advertiser. An advertising network acts as a middleman between the advertiser and the publisher. The advertising network, such as Microsoft adCenter®, may perform dual roles as the advertising network as well as the publisher. One commonly experienced problem with pay per click advertising relates to click fraud. Click fraud is the practice of artificially inflating traffic statistics to defraud the advertiser or the publisher. By using an automated clicking program or employing low-cost workers to click the link to a target advertisement, perpetrators of the click fraud scheme create an illusion that a large number of potential customers are clicking a link to the target advertisement when, in fact, there is no likelihood that any of the clicks will lead to profit for the advertiser. A sizable portion of advertising budget is lost due to click fraud.
Therefore, the advertiser, publisher and advertising network have worked to develop a better way of detecting click fraud. Owing to their efforts, abnormally high number of clicks coming from one or a small group of computers can be easily detected. This eliminates small scale click fraud activities. However, a large scale click fraud operation is often based on a script which simulates a human clicking from diverse geographic locations. In order to stop this type of click fraud, users' data for clicked advertisements are often necessary in analyzing the users' web activities to detect the presence of click fraud. However, the scope of the users' data available to the publisher may not be enough to identify patterns that span several advertisers. This is due, in part, to the fact that the publisher, unlike the advertising network, often deals with a small number of advertisers. Even with a larger set of user data available in the advertising network, a conventional analysis of the user data may not be enough to detect the click fraud operation since the user data is often based on a script which is intentionally designed to simulate real human clicking behaviors. Unfortunately, click fraud remains a serious problem facing online advertisers.